DfR Solutions' Insights

There are certain reliability tools available for thermal testing that have the ability to provide accurate predictions for even very complex thermal management solutions. But, they fall short of determining how hot is too hot for electrolytic capacitors.

It’s not that electrolytic capacitors are immune to thermal testing – the opposite, in fact, is true – but rather that classic de-rating is no longer satisfactory within the electronics industry. Its broad assumptions, which are not based on actual failure modes and degradation mechanisms, can result in overly conservative and expensive designs or products with insufficient reliability.

Knowing de-rating no longer provides optimal value, engineers have devised a better approach that uses electrolytic capacitors’ thermal modeling and measurement data within design rules and predictive tools that are based on reliability physics.

Electrolytic Capacitor Construction

Electrolytic capacitors are the only components that rely on liquid for functional operation. As such, the electrolytic capacitors have a limited lifetime due to the gradual evaporation of their liquid electrolyte. This loss leads to decreased capacitance and increased equivalent series resistance (ESR), so all electrolytic capacitor manufacturers provide a rated lifetime.

Manufacturer Ratings and Lifetime De-rating

Most power supply engineers extrapolate manufacturer ratings using the Arrhenius equation that takes rated lifetime, rated temperature and ambient temperature to arrive at a conservative prediction of electrolytic capacitor lifetime.

Since the actual 1 to 5% failure rates at lifetime are more modest than MTTF, designers routinely apply a 50% de-rating to the ripple current to greatly extend electrolytic capacitor lifetime.

Mixed Results in Testing and Calculations

A recent push to lower cost and reduce dimensions has caused designers to be more aggressive in testing, applying ripple currents that exceed manufacturer ratings for shorter durations. The result? Designers squeeze out more margin by taking temperature changes into consideration when calculating electrolytic capacitor lifetime.

Actual lifetime can also vary depending on the sensitivity of the circuit to change in component parameters. Circuit sensitivity could induce product failure well before capacitor failure. When dimensional constraints force designers to place electrolytic capacitors adjacent to, or touching, hot components, the standard lifetime equation may not be applicable. The non-uniform temperature distribution across the electrolytic capacitor may induce accelerated degradation and increase pressure to the point of rupture.

Sherlock Automated Design Analysis™ Software

The variables in electrolytic capacitor testing and lifetime calculations, coupled with the proven imprecision of de-rating, make it even more important for power supply engineers to seek predictive tools that are based on reliability physics.

One such tool is Sherlock Automated Design Analysis™ Software. Sherlock uses Physics of Failure (PoF) to identify when thermal temperatures are too hot for electrolytic capacitors, and find solutions based on real-life data inputs and complex calculations performed in comprehensive software databases. The results are tabulated in a matter of minutes, not days, and are available well before prototyping starts – saving money and time.

Find out more about reliability physics in our case study, Reliability Modeling Software Helps Designers Get a Jump on Testing. Click the button below to download the slide presentation or watch the webinar on demand now.